1. Field of the Invention
The present invention relates to a switching power source apparatus that correctly detects a peak value of an AC voltage, and if the AC voltage is too low, stops the switching operation of a switching element to protect the switching element from breakage. Such a breakage may occur during an abnormal test of the switching power source apparatus, such as a brownout test that gradually decreases an AC voltage applied to the switching power source apparatus from a rated voltage and a brown-in test that gradually increases an AC voltage applied to the switching power source apparatus from a low voltage to the rated voltage. The breakage of the switching element may also occur with a poor power source that applies a low voltage to an AC input terminal of the switching power source apparatus, to cause the switching element to excessively increase its ON duty.
2. Description of the Related Art
There are some related arts disclosing switching power source apparatuses that include a switching element breakage protection to prevent a switching element from excessively increasing an ON duty when an AC voltage from an AC/DC converter decreases. These related arts detect an input voltage, and if the input voltage drops below a threshold value, determine that it is a low input state to stop the switching element.
FIG. 1 is a circuit diagram showing a switching power source apparatus according to one of such related arts. The apparatus includes an input rectifying-smoothing circuit 10, a transformer 20, a primary winding 21, a drive winding 23, a starter 30, a rectifying-smoothing circuit 40, a low-voltage-malfunction preventer 50, a reference voltage circuit 60, a feedback controller 70, a switching element 80, a current detecting resistor 90, a low-pass filter (LPF) 100, a first oscillator 110, a PWM controller 130, a NOR gate 140a, a driver 150, a comparator 160, a low-input protector 200a, a delay circuit 210a, a rectifying-smoothing circuit 170, an output voltage detector 180, and a load 190.
An end of the primary winding 21 of the transformer 20 is connected through the starter 30 and rectifying-smoothing circuit 40 to the drive winding 23 of the transformer 20.
The low-input protector 200a protects the switching element 80 from breakage due to a low voltage that is applied to an AC input terminal and excessively expands an ON duty of the switching element 80. Such a low voltage is applied during a brownout test or a brown-in test, or from an inferior power source.
The low-input protector 200a includes voltage dividing resistors 201 and 202 to divide a DC input voltage V2, a capacitor 208 to average AC ripple voltage superimposed on the DC input voltage V2, and a comparator 203 to compare a DC input divided voltage V16 with a threshold value V17.
The configuration and operation of each part of the related art shown in FIG. 1 will be explained with reference to the circuit diagram of FIG. 1 and a timing chart of FIG. 2. The input rectifying-smoothing circuit 10 includes diodes 11 to 14 to rectify an AC voltage V1 of an AC power source and a smoothing capacitor 15 to smooth the rectified voltage and provide a DC input voltage V2. The voltage V2 is supplied through the primary winding 21 of the transformer 20 to the switching element 80.
The switching element 80 conducts a switching operation, i.e., turns on/off to convert the DC input voltage V2 into an AC voltage to transfer energy from the primary side of the transformer 20 to the secondary side thereof. The rectifying-smoothing circuit 170 rectifies and smoothes a voltage generated by a secondary winding 22 of the transformer 20, to supply power to the load 190.
When the DC input voltage V2 increases to a breakdown voltage VBD of a Zener diode 31 in the starter 30, the Zener diode 31 breaks down to pass a starting current I5. The starting current I5 charges a capacitor 42 in the rectifying-smoothing circuit 40, to increase a power source voltage V3.
When the power source voltage V3 reaches a first threshold value V25a of the low-voltage-malfunction preventer 50, the reference voltage circuit (REG) 60 operates to supply a reference voltage V4 to each circuit block, and at the same time, turn off a switch 33 in the starter 30 to stop the starting current I5.
Operation when the AC voltage V1 increases from a low state as an inoperative state for power stoppage will be explained. In the low state, the DC input voltage V2 includes substantially no AC ripple voltage caused by the load 190.
In the low-input protector 200a, the voltage dividing resistors 201 and 202 divide the DC input voltage V2, to detect a DC input divided voltage V16 that is proportional to the AC voltage V1. The comparator 203 compares the DC input divided voltage V16 with a first threshold value V17a. If the DC input divided voltage V16 is equal to or larger than the first threshold value V17a, the comparator 203 determines that it is a steady input state, switches a first stop signal V18 from high to low, and provides the NOR gate 140a with the low-level signal V18, so that the switching element 80 may start the switching operation for a steady operation period.
Operation when the AC voltage V1 decreases from a high state as an operative state will be explained. In the power source operating state, the DC input voltage V2 includes superimposed ripple voltage that is caused by a primary-side load current and is synchronous with a period of the AC voltage V1. The AC ripple voltage varies in response to a secondary-side load current I2 and increases as the load current I2 increases. The DC input voltage V2 including the AC ripple voltage is divided by the voltage dividing resistors 201 and 202 into the voltage V16.
The comparator 203 compares the DC input divided voltage V16 averaged by the capacitor 208 with a second threshold value V17b. If the DC input divided voltage V16 is below the second threshold value V17b, the comparator 203 determines that it is a low input state and provides the NOR gate 140a with a high-level stop signal V18 to stop the switching operation of the switching element 80.
Another of the related arts is disclosed in Japanese Unexamined Patent Application Publication No. 2005-328589. This is a switching regulator control circuit including a first low-voltage-operation stop circuit to detect a first voltage value of input voltage and a second low-voltage-operation stop circuit to detect a second voltage value lower than the first voltage value. Depending on outputs from the first and second low-voltage-operation stop circuits, the switching regulator control circuit stops operation. One of the first and second low-voltage-operation stop circuits always operates to monitor a voltage value of input voltage.